Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual nação possui o veleiro USA-18?
 ###Tabelas: CREATE TABLE total_playoff_berths_while_in_the_afl_afc_east (total_playoff_berths_while_in_the_afl_afc_east_id AUTO_INCREMENT PRIMARY KEY, Team VARCHAR, Division Titles INT, Playoff Berths INT, AFL Titles INT, AFC Championships INT, Super Bowl Championships INT) CREATE TABLE result_summary (result_summary_id AUTO_INCREMENT PRIMARY KEY, Opposition VARCHAR, Span VARCHAR, Lost INT, Tied INT, Win % INT) CREATE TABLE competing_yachts (competing_yachts_id AUTO_INCREMENT PRIMARY KEY, Sail VARCHAR, Yacht VARCHAR, Syndicate VARCHAR, Yacht Club VARCHAR, Nation VARCHAR) CREATE TABLE broadcast_schedule (broadcast_schedule_id AUTO_INCREMENT PRIMARY KEY, Episode VARCHAR, Date VARCHAR, Time VARCHAR, Network VARCHAR, Length VARCHAR) CREATE TABLE leading_point_scorers (leading_point_scorers_id AUTO_INCREMENT PRIMARY KEY, Player VARCHAR, Span VARCHAR, Start INT, Tries INT, Conv INT, Pens INT, Drop INT)  
SELECT Nation FROM competing_yachts WHERE Sail = usa-18